1. Field of the Invention
The present invention relates to an exposure method suitable for an application to an exposure apparatus for transferring patterns on a reticle onto a photosensitive substrate when manufacturing, e.g., a thin-film magnetic head or a semiconductor device, etc. by a photolithography process.
2. Related Background Art
Manufacturing, e.g., a thin-film magnetic head or a semiconductor device by a photolithography process has hitherto involved the use of a projection exposure apparatus. The projection exposure apparatus projects patterns on a photomask or a reticle (hereinafter generically termed a [reticle]) onto a substrate (a semiconductor wafer, a ceramics plate, a glass plate, etc.) coated with a photosensitive material such as a photoresist or the like through a projection optical system. For instance, an area corresponding to a predetermined pattern on a predetermined substrate is required to be cut off. In this case, an image of the predetermined pattern is projected on the substrate coated with the photoresist through the projection exposure apparatus. Thereafter, the photoresist on the area corresponding to the image of the predetermined pattern is removed by a developing process. Thus, only a substrate surface of the photoresist-removed area can be cut by an etching equipment.
Recently, a dry etching equipment has been employed as the etching equipment. The dry etching equipment physically cuts the surface by partially colliding the photo-resist-removed substrate with atoms like argon (Ar), etc. When the substrate surface is etched on the order of several .mu.m to several tens .mu.m by use of the etching equipment described above, it is required that the photoresist be also on the order of several tens .mu.m thick. It is because a selection ratio of the photoresist serving as a protection film to the substance on the substrate surface is smaller than in the case of performing etching with the aid of a reactive gas.
FIG. 8 illustrates a profile of the substrate when etched in such a manner. Referring to FIG. 8, the surface of the substrate 1 is covered with a photoresist 2. The photoresist corresponding to a pattern 3 to be etched is removed. Further, side surfaces (hereinafter simply called [edges]) 3a of the photoresist 2 corresponding to edges of the pattern 3 are substantially perpendicular to the surface of the substrate 1. For example, argon atoms 4 fall on the substrate 1 from above of the pattern 3, with the result that a substance of the substrate 1 at the bottom of the pattern 3 is cut off. A recess 5 is thus formed. As illustrated in FIG. 8, however, if the edges 3a of the photoresist are steeply inclined, fragments of the cut-off substance 6 of the surface of the substrate 1 are adhered to the edges 3a of the photoresist. The adhered substance may turn out to be an obstacle against the etching effected afterward in some cases. Making the inclinations of the edges 3a gentle is effective in preventing the adhesion of the substance of the surface of the substrate 1.
FIG. 9 shows an example where the edges have gentle inclinations. Referring to FIG. 9, edges 7a of a pattern 7 in the photoresist covered on the substrate 1 are inclined to the substrate 1. For this reason, even when the argon atoms 4 collide with the substrate via the pattern 7, the fragments of the surface substance of the substrate 1 are not adhered to the edges 7a and do not.
A method of forming the edges 7a having the gentle inclinations as shown in FIG. 9 in the photoresist 2 may be a so-called defocus method. The defocus method is a known method of exposing a reticle pattern by hitherto shifting the substrate 1 from a focal position of a projection optical system of a projection exposure apparatus. A developing process is effected on the photoresist 2 on the substrate 1 undergoing an exposure based on the defocus method described above. The photoresist is thereby removed in a profile as illustrated in FIG. 9. Further, setting a defocus quantity to a predetermined magnitude has hitherto involved the use of an autofocus sensor for detecting a quantity of positional deviation of an exposure plane of the substrate 1 from a best image forming plane of the projection optical system.
If a width of the pattern projected on the substrate is larger than a resolving power of the projection optical system according to the defocus method, however, a remarkably large defocus quantity (e.g., 100 .mu.m or above) is needed. There exists a possibility of exceeding a follow-up range of the autofocus sensor. This is conducive to the following drawback. When transferring a pattern having a large width onto the substrate, it is difficult to make gentle the inclinations of the edges of the photoresist-removed areas after the developing process.